Water in macromolecular models

In PDB files resulting from X-ray crystallography that do specify water positions, typically only 10-20% of the water that was actually present in the crystal is shown. Protein crystals used for X-ray diffraction are about half water, but the majority of the water present is disordered and cannot be resolved. Only tightly bound, stationary water molecules can be experimentally resolved in the electron density map. Similarly, in PDB files resulting from NMR, only tightly bound water can be resolved, and the majority of the water is not represented.